Li-ion uptake and increase in interlayer spacing of Nb4C3 MXene

Two-dimensional (2D) carbides, MXenes, have shown promise for electrochemical energy storage, but most studies were conducted on titanium carbides. Herein we report on Li insertion into a 2D Nb4C3Tx MXene, formed by etching aluminum from Nb4AlC3 in HF at room temperature. The theoretical capacity of Nb(4)C(3)Tx has not been studied, but it displayed a higher capacity than as-produced titanium carbide and other MXenes, when tested as anode for lithium ion batteries. In addition, the charge/discharge capacity of the Nb(4)c(3)Tx anode increases with cycling. For instance, after 100 charge/discharge cycles, the specific capacity increased from 310 mA h g(-1) (194 mA h cm(-3)) to 380 mA h g(-1) (238 mA h cm(-3)), at a current density of 0.1 A g(-1), and the capacity increased from 116 mA h g(-1) (73 mA h cm(-3)) to 320 mA h g(-1) (200 mA h cm(-3)) at 1 A g(-1). Excellent rate capability and superior long-term stability at the ultrahigh rates have been demonstrated. This work is expected to inspire further exploration of MXenes for high-performance Li-ion batteries and capacitors. Since Nb(4)C(3)Tx is just one of many MXenes, there is much room for improving the accessibility of the electronically conducting layered structures of MXenes and achieving a better electrochemical performance.